Optical waveguide couplers of various constructions are well known and in widespread use, especially in the telecommunications field. Generally, such optical waveguide couplers make it possible to interconnect individual optical waveguides so that the modulated light propagating through an input optical waveguide leading to the optical waveguide coupler continues to propagate through at least one output optical waveguide leading from the optical waveguide coupler. In some optical waveguide couplers, at least two input optical waveguides are fused together within the optical waveguide coupler so that the output light signal is a combination of the input light signals. Examples of optical waveguide couplers are disclosed by U.S. Pat. No. 4,902,324 to Miller et al., U.S. Pat. No. 4,931,076 to Berkey, U.S. Pat. No. 4,948,217 to Keck et al., and U.S. Pat. No. 4,943,130 to Dannoux et al.
Optical waveguide couplers perform quite well. However, they are sensitive to adverse environmental effects. For instance, problems can result from the coupling region of the optical fibers usually being bare (i.e. not provided with a jacket). In addition, optical waveguides are susceptible to bending or breakage where they are connected to couplers. As a result, any significant movement of one end of the coupler with respect to the other end results in an order of magnitude change in performance. The couplers thus have to be packaged in a strong, stiff material which prevents such movement and allows the coupler to be handled and installed in normal environmental conditions without substantial performance degradation.
There are many strong, stiff materials which have been utilized to package optical waveguide couplers, including metals and polymer compositions. For example, optical waveguide couplers have been packaged in a housing made from polycarbonate or other engineering plastics. Such packaging permits easier handling, provides protection against mechanical shock-like vibrations, and imparts environmental protection against temperature and humidity variations.
U.S. patent application Ser. No. 593,903, entitled "Method For Encapsulating an Optical Component and the Encapsulated Component Obtained Thereby" to Dannoux couples optical waveguides with a bar of glass encapsulated by a casing. Free space between the bar and the casing is filled with a sealing composition (e.g., epoxy resin or solder).
U.S. Pat. No. 4,707,069 to Hoffman III relates to an optical waveguide coupler with a V-shaped cross-section bounding an open channel for the fiber. The V-shape is formed by a metal support having a coefficient of thermal expansion approximately equal to that of the optical waveguide. Within the coupler, the optical waveguide's coating has been removed, and the fibers are held in place with an adhesive.
U.S. Pat. No. 4,906,068 to Olson et al. relates to an optical waveguide coupler with a housing made of quartz glass.
Japanese Published Patent Application No. 03-045911 discloses a housing for a branched coupler of optical waveguides produced from a composition of an anisotropic polymer resin and a fibrous filler with a low coefficient of thermal expansion. The filler is produced from carbon fibers or organic high molecular weight fibers. The resulting composition has a coefficient of thermal expansion of about 1.times.10.sup.-6 .degree. C..sup.-1. Injection molding is used to form the housing around the coupler; however, injection molding is a high temperature operation which can damage optical fibers connected by the coupler. Further, such molding operations tend to shorten the carbon or organic fibers in the housing composition to lengths that are too short to impart strength to the coupler along a substantial part of its length (e.g., 0.18 cm or less).
U.S. Pat. No. 4,482,203 to Stowe et al. discloses an optical waveguide coupler in which waveguide coatings have been removed. The coupler housing is filled with RTV vulcanizing silicones or other filler materials, such as epoxy resins.
Japanese Published Patent Application No. 01-182810 relates to an optical waveguide coupler surrounded by a base plate and box made of plastic.
B. S. Kawasaki, K. O. Hill, and R. G. Lamont, "Biconical-Taper Single-Mode Fiber Coupler," Optical Society of America (1981), B. S. Kawasaki, M. Kawachi, K. O. Hill, and D. C. Johnson, "A Single-Mode-Fiber Coupler With a Variable Coupling Ratio," Journal of Lightwave Technology, vol. LT-1, and T. Bricheno and A. Fielding, "Stable Low-Loss Single-Mode Couplers," Electronics Letters, vol. 20, no. 6 (1984) relate to couplers for optical waveguides in which the coupler is potted with a silicone, epoxy, or gel.
The housing materials disclosed by these references, however, suffer from a number of serious deficiencies.
One problem is that many housings are pre-formed as a rigid tube or an appropriately shaped box into which the coupler is forced. Such force fitting often causes breakage of the coupler or the optical waveguides. Even if no breakage occurs, the coupler or the optical waveguide may be bent resulting in the deleterious impact noted above.
Another problem with prior art coupler housings is that coefficients of thermal expansion of materials in contact with the coupler are not properly matched with that of the coupler material. Where the mismatch is significant, thermal stresses resulting from temperature fluctuations may cause bending of the coupler. This can result in optical property changes or, in extreme cases, breakage of the coupler.
In view of the above-noted problems with packaged optical waveguides, there remains a need for improved packaging systems of this type.